The oldest and still dominating method for drying timber and other wooden products is to place the products in a chamber through which heated air does continuously pass. The warm air flows past the external surfaces of the products and absorbs therefrom moisture which then leaves the chamber together with the air. Accordingly, that method can be defined as a continuous process in an open circuit. It suffers from several disadvantages and limitations, the most important of which are the following ones.
The wooden products are dried by heat conduction in the way that the surface layer is first dried. The drying process does then, at a gradually lower rate, proceed towards the center of each product. This is disadvantageous for several reasons. The drying occurs very slowly, primarily because wood is a very bad heat conductor. In addition thereto, the slow removal of moisture is accentuated by the direction of the moisture gradient. The reason for this is that the drying of the surface layers results in a shrinking, a reduction of the distance between the wood fibres and, hence, in a corresponding restriction of the passages through which the moisture can migrate outwards. This effect is differently pronounced in different sorts of wood and, in many cases, it is accompanied by cracking in the surface layers. For that reason some sorts of wood cannot at all be dried by forced air circulation; instead they require that the drying takes place during a very extended period of time, in extreme cases several years. Due to the fact that the drying occurs in a completely open system the content of excess heat in the air leaving the chamber cannot be used or, stated in other words, the efficiency of the method is very low.
In an effort to avoid that the drying will start at the external surface and then progress inwardly and require long time there has, during the latest years, also been applied another method. According to that method the article to be dried is placed between a pair of electrodes connected to a high-frequency generator. As is well-known, disregarding the edge effects which in this context are completely negligible, the useful electrical field is only present in the space between the electrodes. This means that, for practical and economical reasons, the usefulness of that method is limited to articles the shapes of which are suitable for high-frequency drying and the dimensions of which are relatively small. A third condition is that the articles must be manufactured in long series. This applies to e.g. components used in the furniture industry. Another factual circumstance, also limiting the usefulness of that prior art method, is the low capacity of wood to absorb HF energy. This often results in electrical flash-overs caused by the high electrode voltage necessary. A still further requirement implies that practically no variations in the cross-sectional area of the products can be permitted since, otherwise, they are not dried homogenously and subject to damages by cracking and the like. Accordingly, it is difficult to control such a drying process and it is obvious that it cannot be used for drying logs and the like.